Precision machine vision inspection systems (or “vision systems” for short) can be utilized to obtain precise dimensional measurements of inspected objects and to inspect various other object characteristics. Such systems may include a computer, a camera and optical system, and a precision stage that is movable in multiple directions to allow workpiece inspection. One exemplary prior art system that can be characterized as a general-purpose “off-line” precision vision system is the commercially available QUICK VISION® series of PC-based vision systems and QVPAK® software available from Mitutoyo America Corporation (MAC), located in Aurora, Ill. The features and operation of the QUICK VISION® series of vision systems and the QVPAK® software are generally described, for example, in the QVPAK 3D CNC Vision Measuring Machine User's Guide, published January 2003, and the QVPAK 3D CNC Vision Measuring Machine Operation Guide, published September 1996, each of which is hereby incorporated by reference in their entirety. This type of system is able to use a microscope-type optical system and move the stage so as to provide inspection images of either small or relatively large workpieces at various magnifications.
General purpose precision machine vision inspection systems, such as the QUICK VISION™ system, are also generally programmable to provide automated video inspection. Such systems typically include GUI features and predefined image analysis “video tools” such that operation and programming can be performed by “non-expert” operators. For example, U.S. Pat. No. 6,542,180, which is incorporated herein by reference in its entirety, teaches a vision system that uses automated video inspection including the use of various video tools. Exemplary video tools include edge location tools, which are sometimes referred to as “box tools,” which are used to locate an edge feature of a workpiece. For example, commonly assigned U.S. Pat. No. 7,627,162, which is incorporated herein by reference in its entirety, teaches various applications of box tools.
It is known to some experts in the art of precision machine vision inspection that when sharp edge features (e.g., binary edges) are aligned with an axis of a pixel array of a camera, precision edge location measurements suffer from digitization errors (e.g., due to sub-Nyquist sampling) and subpixel resolution measurements may suffer related measurement errors. In a paper titled “Error Analysis of Subpixel Edge Localization,” by Patrick Mikulastik, et al., the authors approximated a higher resolution sampling rate for straight edges by intentionally rotating a straight edge relative to the pixel columns in a camera, such that various pixel rows “sample” the edge with a slight offset relative to one another. However, precision machine vision inspection systems are often programmed and operated by relatively unskilled users that are not aware of such considerations, and such a technique may not be comprehensible and/or practical for such users. A precision machine vision system that helps unskilled users avoid the errors outlined above would be desirable. A machine vision inspection system configured such that these types of digitization errors are reduced or eliminated without requiring a user to be explicitly aware of such errors would be most desirable.